It depends. If you have apps running, like rss, push, email, etc, it will reduce your battery life. However, your battery life shouldn't be reduced as much as you're showing. I would lose about 10-15% overnight with my 3GS and now, with the iPhone 4, I only lose about 5% overnight. I've noticed a significant increase in battery life with the iPhone 4. If you're seeing that much of an overnight loss, you should see what all you have running because that may be causing an extra drain on your battery. Unless you're receiving massive emails every few minutes, I still don't think you should be seeing a loss like you are.

It's not out of the realm of possibility. Do you have push notifications on, and if so, do you get a lot of emails? That will kill your battery. Also, if you were downloading anything like a movie will obviously use battery power. Leaving location services and bluetooth on all the time can do it an there might also have been an app running in the background that is using a lot of battery power.

not to skirt the issue (and i'm glad others have answered your question helpfully), wouldn't it make sense to keep your phone plugged in overnight? :-)

Even when it's plugged in overnight it stops charging at a full charge. So if he
sleeps for eight hours, but the phone stops charging at hour 5 because it's full,
it's not charging for those last 3 hours.

Even when it's plugged in overnight it stops charging at a full charge. So if he
sleeps for eight hours, but the phone stops charging at hour 5 because it's full,
it's not charging for those last 3 hours.

It's still drawing power and wasting electricity for 3 hours. You can top up the charge at work or in the car.

OP, you may want to try to reset the battery meter. Run it all the way down until the iPhone shuts off and displays the "needs to be plugged in" symbol and then charge it all the way up. See what happens after that.

On my 3GS, I would lose anywhere from 10-20% overnight until I tried shutting off 3G and nothing else, I was able to maintain 95-98% after 8 hours. On the iP4, there are times I would lose just 1% overnight, and on the worse nights it's a mere 3%. (and this with leaving 3G on)

I also usually see less than 5% loss overnight with both push and 3G on. Did you just update Skype recently? It seemed like that cause a 20% drop for me the night after I updated to the newest version with backgrounding.

Do you use a twitter push app? When I use Notifications to push lists or search terms, it works much harder than standard use and drains quickly.

By the way, the first item listed in TiPb's "iPhone 101: How to Maximize iPhone Battery Performance" is:

Be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down)

I went to bed last night at 11pm with a full 100% charge. Left the phone unplugged overnight and woke up at 6:30am with the phone still at 100%. I usually leave it on the charger overnight but tried this just to see how much of a charge it would lose overnight. I have all my email set to fetch each hour and only use Push for ScoreCenter to get final scores of baseball games.

By the way, the first item listed in TiPb's "iPhone 101: How to Maximize iPhone Battery Performance" is:

Be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down)

From what I understand, that doesn't have anything to do with battery life. It's to keep the battery gauge accurate to the actual level of charge. In other words, your gauge may say 90% charge, when in reality it's 80%. A full drain monthly brings the gauge back in line.

For me what I have noticed is I take my iPhone off the charger as soon as it is done charging after light usage the battery will fall to 99% few minutes later then 98%. iPhone doesn't last in the 90's too long in the 80's it does a bit more and then a bit more for the 70's I think this is because of the A4. Also when I leave my iPhone to charge over night I can see that my battery life is greater vs taking it off when as soon as it is fully charged.

I had the same problem: 100% off the charger, powered it OFF, and 50% the next day. Apple said it was bad and replaced it. The second one does not have that problem. I suggest you go to the Genuis Bar and try to have it replaced. BTW my iPhone passed their 'diagnostics,' but the Genius Tech realized the phone was bad.

These batteries work best when topped off whenever possible. Letting it drain down all the way isn't the answer. These are different cell phone batteries than those of yesteryear.

I've just read in the past that letting them drain down all the way isn't good
because when it's fully charged, it won't hold the charge for as long as it could.

This is a complete misconception. Even Apple recommends letting the battery fully discharge until it dies and fully recharging it to 100% at least once a month. These Discharge Cycles help to increase battery life and it's the same process the Battery apps in the App Store go through that promise to increase battery life.

For proper maintenance of a lithium-based battery, its important to keep the electrons in it moving occasionally. Be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).

I find that I can have 100% battery when I go to bed and 7-8 hours later I can have a little as 72% left.

Is it normal to loose that amount of battery while the phone is on standby?

Oh and I have an iPhone 4.

If you were using an iPhone 3GS id say yes. But with the iPhone 4 you shouldnt see that much of a loss. Im not sure if youre one of those people that worry about plugging your phone in overnight or not but thats a simple solution. Also, you may want to try running Battery doctor and see if that makes any difference.

This is a complete misconception. Even Apple recommends letting the battery fully discharge until it dies and fully recharging it to 100% at least once a month. These Discharge Cycles help to increase battery life and it's the same process the Battery apps in the App Store go through that promise to increase battery life.

Agreed. If im not mistaken batteries like this have "memory" so letting them drain every once and a while is good because if you are always plugging it in when its say 90 all the time then the times when its real low it may not put as much of a charge in it. something like that. Been a while since ive read up on that issue